Yeah well that sailfin plec they sold you is probably a gibbiceps, which although is peaceful and all, will grow to a 12-18" destroyer of plants and your aquascape, for your size tank you want a bristlenose.
Don't worry so much about lowering the tank's temp that much, just set the thermostat down and the temp will drop by itself unless your room is warm then it'll only go down to ambient which could be around 28-30c atm. You might also want to turn the flow down on your filter or deflect it off the back glass, gourami's come from still water and don't do well in a lot of flow hence why that one is hiding behind the filter.